What's the difference between the Subdivision Surface modifier and the edit mode->RMB->Subdivide option?
"Modifiers are automatic operations that affect an object’s geometry in a non-destructive way. With modifiers, you can perform many effects automatically that would otherwise be too tedious to do manually (such as subdivision surfaces) and without affecting the base geometry of your object."
https://docs.blender.org/manual/en/latest/modeling/modifiers/introduction.html

If you want to preserve the mesh for ease of use, Modifiers are the way to go. if you don't care about it they are not that different except more options before applying the modifier